Contents

The modern kilogram has its origins in the pre-French Revolution days of France. In 1790 an influential proposal by Talleyrand called for a new system of units, including a unit of length derived from an invariable length in nature, and a unit of mass (then called weight) equal to the mass of a unit volume of water.[1] In 1791, the Commission of Weights and Measures, appointed by the French Academy of Sciences, chose one ten-millionth of the quarter meridian as the unit of length, and named it metre.[2][3] Initially a provisional value was used, based on the old meridian measurement by Lacaille (1740).[4]

In 1793 the commission defined the unit of mass as a cubic decimetre of distilled water at 0°C, and gave it the name grave.[5] Two supplemental unit names, gravet (0.001 grave), and bar (1000 grave), were added to cover the same range as the old units, resulting in the following decimal series of units: milligravet, centigravet, decigravet, gravet, centigrave, decigrave, grave, centibar, decibar, bar.[6][7] The mass of a unit volume of water at 0°C was accurately determined by Lavoisier and Haüy (18841 grains per cubic provisional decimetre). A prototype of the grave was made in brass.

In 1795 a new law replaced the three names gravet, grave and bar by a single generic unit name: the gram.[8] The new gram was equal to the old gravet. Four new prefixes were added to cover the same range of units as in 1793 (milligram, centigram, decigram, gram, decagram, hectogram, kilogram, myriagram).[9][10] The brass prototype of the grave was renamed to provisional kilogram.

In 1799 the provisional units were replaced by the final ones. Delambre and Méchain had completed their new measurement of the meridian, and the final metre was 0.03% smaller than the provisional one. Hence the final kilogram, being the mass of one cubic decimetre of water, was 0.09% lighter than the provisional one. In addition, the temperature specification of the water was changed from 0°C to 4°C, the point where the density of water is maximal. This change of temperature added 0.01% to the final kilogram.[11][12] In 1799 a platinum cylinder was made that served as the prototype of the final kilogram. It was called the Kilogramme des Archives as it was stored in the Archives Nationales in Paris. This standard stood for the next ninety years.